1. Field of the Invention
This invention relates to a stepping motor and a magnetic disc reproducing apparatus using the same.
2. Description of the Prior Art
An electronic still camera using a magnetic disc in lieu of a conventional film using a silver halide photographic material as a photosensitive material, and an apparatus for reproducing picture information recorded by the electronic camera on such a magnetic disc are proposed now as one form of magnetic recording and reproducing apparatus.
In such an electronic still camera (a recording apparatus) using a magnetic disc as a recording medium, a magnetic head is disposed opposite to the magnetic disc in a relation closely adjacent to or making contact with the magnetic disc rotating at a high speed. This magnetic head is advanced for stepwise scanning operation each time the magnetic disc makes one revolution, so that many picture information can be radially recorded on concentric tracks.
In the playback mode reproducing the many picture information recorded on the concentric tracks of the magnetic disc, a stepping motor is commonly used as a means for driving a magnetic head in a reproducing apparatus. That is, when a specific number of pulses corresponding to the pitch between the tracks are applied to the stepping motor, the magnetic head is moved from the position on one track to that on the next track. Also, tracking control can be attained according to a so-called climbing servo action based on a reproduced envelope detected by the magnetic head.
One form of such a reproducing apparatus will be described with reference to FIGS. 9 to 11. referring to FIGS. 9 to 11, a lead screw shaft 2 forming part of a scanning mechanism is rotatably journalled in bearings 3 on a bottom plate of a casing 1 of the magnetic disc reproducing apparatus. A pre-loading spring 4 is interposed between the lead screw shaft 2 and one of the bearings 3 to absorb any play of the bearings 3. This lead screw shaft 2 is formed at its middle part with an externally threaded portion 5 having an effective length larger than the scanning length of a magnetic head 19 (described later), and a pair of bushes 6 are fitted on both end portions of the lead screw shaft 2 to provide a sliding-movement guide having a diameter larger than that of the externally threaded portion 5. A gear 7 is mounted on one end (the left-hand end in FIG. 10) of the lead screw shaft 2 to rotate the lead screw shaft 2, and a head-driving stepping motor 9 provided with a pinion 8 is mounted on a side plate of the casing 1. An idle pinion 10 and an idle gear 11 are coaxially and integrally mounted between the gear 7 and the pinion 8 as a drive force transmission means, so that drive force of the stepping motor 9 can be transmitted to the lead screw shaft 2.
A scanner arm 12 is slidably mounted at a part adjacent to its base end on the bushes 6 which act as the guide guiding the sliding movement of the lead screw shaft 2 therealong. In order to prevent rotating movement of the scanner arm 12 around the bushes 6, a hole 13 penetrates vertically through a portion of the base end of the scanner arm 12 remote from the lead screw shaft 2, and a steel ball 14 fitted in this hole 13 is pressed by a leaf spring 14' upward onto an associated portion of the casing 1. Thus, a counter-clockwise reaction force is produced in the scanner arm 12, and a roller 15 interposed between a portion of the scanner arm 12 relatively near the free end thereof and an associated portion of the casing 1 bears this reaction force.
The scanner arm 12, which is slidable in the scanning direction only, has, at its lower surface between the bushes 6, a leaf spring 16 fixed by machine screws thereto in a relation parallel to the lead screw shaft 2, and a needle holder 17 holding a needle 18 thereon is fixed by a machine screw to this leaf spring 16, so that the needle 18 engageable with the externally threaded portion 5 of the lead screw shaft 2 can be guided along the threaded portion 5 and brought into intimate pressure engagement with the threaded portion 5.
A magnetic head 19 is mounted on the free end portion of the scanner arm 12. This magnetic head 19 is located opposite to a magnetic disc 22a contained in a magnetic disc pack 22 which is detachably mounted on a magnetic disc drive motor 20 installed in the casing 1 and which is positioned by positioning pins 21. A counter 23 is provided on the pack 22.
In the prior art reproducing apparatus described above, picture information recorded on a desired track is commonly reproduced by returning the magnetic head 19 once to the position on an outermost or corresponding track (referred to hereinafter as a track No. zero) and then moving the scanner arm 12 by the stepping motor 9 until the magnetic head 19 is positioned on the desired track. For this purpose, a doctor 24 is provided on the scanner arm 12, and a limit switch 25 is provided on the casing 1 as shown in FIG. 10. It has been decided that, when the doctor 24 contacts the limit switch 25, the magnetic head 19 has returned to the position on the track No. zero.
As is well known in the art, the interval or pitch between the tracks formed on the magnetic disc 22a is 100 .mu.m, and the track width and the guard band width are 60 .mu.m and 40 .mu.m respectively. This means that an accuracy as high as the order of microns is required for mounting the limit switch 25. Therefore, a long period of time has been required for adjusting the position of the limit switch 25, and yet it has been difficult to fully accurately adjust the position of the limit switch 25.